The adapter nut takes the place of the prop nut and lock nut that comes with the engine.
If you notice, the adapter comes with a new prop washer, and the adapter nut has a shoulder that fits inside the washer when you are tightening down the prop. To understand, just hold the prop washer and adapter nut in your hand. Fit the shoulder on the adapter nut into the hole of the prop washer and you'll see what I mean............
Take the engine's original prop washer, nut and lock nut and put them back in the engine box.........use the new prop washer and adapter nut to tighten down the prop to the back plate and the engine. So far, I haven't had a problem with my 125 back firing the prop off the engine with my new Dave Brown Spinner set up. I like mine....................